Bev Oke Ian Powell Denis Laurin NRC Herzberg Institute of Astrophysics Victoria, BC, Canada

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Bev OkeIan PowellDenis LaurinNRC - Herzberg
Institute of Astrophysics Victoria, BC, Canada
HIA TMT WFOS Concept
IWG Mar 17-18, 2004 Santa Cruz
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HIA TMT WFOS Concept

• Main requirements and design approach
• Initially TMT R-C 30 m, F/15 ? Also Gregorian.
• Natural seeing 0.25 ? 0.75.
• 20 arcmin field or 2.6 m diameter (for F/15).
• 0.310 to 1.1 um wavelength coverage ? red blue
  cameras.
• Multi-object, cover 100s object simultaneously ?
  slit mask.
• Imaging.
• Use Nasmyth platform, fixed gravity vector
  mounting, image rotation platform.
• Reflective vs refractive pupil too large for
  refractive elements?
• Max. field ? partition 4-instrument layout for
  maximum field coverage.

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HIA TMT WFOS Concept

• Quote from SRD
• Wavelength range 0.3 1.3µm (goal) 0.31 1.0µm
  (required).
• The goal is to record the entire wavelength range
  in a single exposure. However, for optimized
  performance this wavelength range can be covered
  through two optimized arms covering two
  wavelength ranges. The wavelength split should be
  near 0.7µm.
• Spectral resolution R 300-5000 (requirement)
  for a 0.75 arc-sec slit
• Field of View 10 arc-min, with the intent of
  covering 75 square arc-min (requirement). 300
  square arc-min field (goal)
• The field need not be contiguous.
• Throughput Comparable to LRIS, GMOS, et al.
  (notionally greater than 30 from 0.31 1.0µm).
• Image quality
• Imaging Less than 0.2 arc-sec FWHM over any 0.1
  µm interval (goal). This includes the
  contribution from the ADC.
• Spectroscopy Less than 0.2 arc-sec FWHM at any
  wavelength.
• Spatial sampling (per pixel) lt 0.1 arc-sec
  (goal) lt 0.15 arc-sec (requirement)
• Desirable features
• Possible cross-dispersed mode for smaller
  sampling density and higher R.
• Imaging through narrowband filters.
• IFUs

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Design options system overview
Telescope
R/C, Fprimary, Feff
Greg, Fprimary, Feff
Initial point design
In favor by others
WFOS
Fore-optics
Cameras
Collimator
?
Reflective fore-optics
Lens based on Epps
Limited to 360 mm
Reflective re-imaging
Lens system
Catadioptric
Favored
Up to 1 m?
Lens re-imaging
Catadioptric
Too complex
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Telescope

• Telescope Models (30 m, 20 field)

Data from excel file data on telescope parameters
with equations. Zemax models created for all.
from Sys Eng Work Group.

• Effect of type curvature of image surface.
  R_curv. depends on F/primary and secondary.
• Effect of F/eff scaling of relay optics, less
  effect on rest of optics.

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Telescope

• CELT (30 m, 20 field)

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Telescope

• RMS Spots (30 m, 20 field)

• Box 1 arcsec
• Curved image surface

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Early Design Concepts

• Spectrograph relay optics re-imaging

• Recall Bev Okes concept.
• Approach relay optics, re-imaging using LRIS
  type lenses.
• Evolution mostly reflective optics.

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Design Options

• Spectrograph relay optics re-imaging

• Large (1.5 m) collimating reflector (no choice?).
• Re-imaging (reduced scale, typically 0.4x)

• Reflective, 3-mirror off-axis design
• Requires more space to avoid obstruction of
  beams.
• No chromatic aberrations.
• RMS spot size below 50 um possible.
• Conic surfaces, but not aspheric.

• Refractive using LRIS type camera
• Works hard to cover spectral range.
• Needs CaF2, aperture limited to available glass
  (lt 360 mm?).
• Up to 14 elements.
• RMS spots not better than 100 um (3 aspherics).

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Relay optics with lens

• Spectrograph relay-optics re-imaging layout

• Lens system based on LRIS camera.
• Re-optimized to cover 0.31 to 1 um.
• Scale reductions 0.207.
• 14 elements, 280 mm max diameter.
• 3 aspheric surfaces.
• Telecentric.
• More compact than 3-mirror design.

File HTS_CELTLRESblue_tilt5-12fields-reopt.ZMX
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Relay optics with lens

• Spectrograph relay optics re-imaging spots

• Box 1 arcsec(450 um).
• Field 4 min square.
• (3.73, 3.73) to (7.73, 7.73)

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Resolution problem

• Spectral Resolution
• Science requirements R5000 with 0.75 slit
  width.

• John Pazder has been investigating this issue I
  have been investigating the R5000 / 0.75"
  specification impact on the WFOS design with a
  DEIMOS type camera (refractive with CaF2, so
  limited to lt 360mm diameter) and ruled reflection
  gratings. To achieve R5000 the grating must be
  inclined to the camera, an Echelle type
  arrangement except with a significant deviation
  angle (45 degrees), rather than the standard
  grating normal to the camera arrangement is
  required. The primary disadvantage identified
  for this arrangement is the large blaze angles (gt
  45 degrees) impact on grating efficiency in the
  first order, in particular toward 10000 A where
  the efficiency is very poor. Polarization
  effects are particularly strong on the red end as
  well. The alternatives VPH and immersion
  gratings are being investigated.

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Relay optics with 3-mirror design

• Spectrograph collimator (after re-imaging, slit
  mask)

• Catadioptrics (Ian Powells design, coming up)
• No chromatic aberration problem
• Up to 1 m aperture, larger camera possible for
  higher resolution.
• Requires only quartz lenses.
• Conic surfaces, but non aspheric surface
  possible.
• For rotationally symmetrical systems, have a
  central obscuration

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Relay optics with 3-mirror design

• Relay optics - 3 mirror anastigmat (no aspherics)

Square Telescope image surface
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Relay optics with 3-mirror design

• Relay optics - 3 mirror anastigmat RMS spots.
• Image scale 0.35x telescope.
• Image surface curvature -4.6 m.
• Box 1 arcsec.

• Some preliminary tolerance analysis on 3-mirror
  (criteria RMS x 2 on slits image surface
• Tilts lt 0.1deg or so.
• Clocking not sensitive.
• Dec a few mm.

Square field limits (1.5,1.5) to (6,6).
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Schematic of collimator
Collimator reflector approach

• 1 m diameter optics
• F 5 m

Fold mirror
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Collimator reflector approach
Collimator Aberration curves
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Schematic of camera
Camera reflector approach

• 1 m diameter optics
• F 2.1 m

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Camera aberration curves
Camera reflector approach
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WFOS reflective optics
Whole spectrograph optics
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HIA WFOS reflective optics
Whole spectrograph optics. Two possible
arrangements.
Telescope image surface
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HIA WFOS reflective optics
30M R-C telescope spectrograph
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HIA WFOS reflective optics
30M R-C (CELT) telescope spectrograph(Another
view)
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HIA WFOS reflective optics
30M R-C telescope spectrograph optics spot
diagrams

• Wavelengths
• 430, 310, 550 nm
• Includes grating dispersion but superimposed.
• Worst RMS spot 0.1 arcsec

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HIA WFOS reflective optics
30M R-C telescope spectrograph optics spot
diagrams
Wavelengths Zemax analysis, Box 1 arcsec
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Projected image on detector 6.9x reduced from
Nasmyth focus
HIA WFOS reflective optics
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Comparison of different front-end telescopes
Telescope Options

• Image radius and exit pupil location of
  telescope
• Telescope type Image Radius (mm) Exit pupil
  location (mm)
• F/15 Ritchey-Chretien 6000
  (concave) -61000
• F/12 Ritchey-Chretien 7000
  (concave) -61000
• F/20 Ritchey-Chretien 4500
  (concave) -61000
• F/15 Gregorian (F/1.25) 4700 (convex) -54000
• F/18 Gregorian (F/1.0) 2700
  (convex) -54000
• Image radius and exit pupil location of
  telescope/3mirror combination
• Telescope type Image Radius (mm) Exit pupil
  location (mm)
• F/15 Ritchey-Chretien 4500 (concave)
  -5500
• F/12 Ritchey-Chretien 6100
  (concave) -10000
• F/20 Ritchey-Chretien 4400 (concave)
  -3100
• F/15 Gregorian (F/1.25) 5500 (convex) -5000
• F/18 Gregorian (F/1.0) 7200 (convex) -1250
• Image plane tilt of around 5 degrees with
  respect to optical axis

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3 m anastigmat for F/15 Gregorian telescope
F/1.25 primary
Gregorian Option
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3 m anastigmat for F/15 Gregorian telescope
F/1.25 primary Spot diagrams
Gregorian Option

• Image scale 0.316 mm/arcsec
• Ref circle 0.79 arcsec
• Worst RMS spot 0.13 arcsec

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Design Options
Re-design of collimator optics and camera
objective(Ian Powell)

• Collimator
• can be re-designed to handle exit pupil location
  for any of the configurations with exit pupils
  further away than 3000 mm, however, radius of
  image cannot be controlled simultaneously.
• Camera objective
• should be possible to re-design camera objective
  to flatten residual image curvature at detector
  of greater than /-3000 mm.
• Further re-optimization of these two system
  groups indicates solutions can be obtained with
  fewer elements.

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Multi-instrument Options

• Spectrograph mosaic options

• 2-instrument or 4-instrument.
• Compactness limited to overlapping of optics.
• FOV area, depends on min/max field w.r.t. to
  optical axis.
• Blue and Red camera on all? (cost complexity
  issue)
• Center of mosaic is free for other instrument.

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Multi-instrument Options

• 4-instrument layout possibility (fore optics
  only)

Square Telescope image surface
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2
3
Square field limits (1.5,1.5) to (6,6) each.
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Design Options

• Short note on coatings
• Mirrors Keck coating shows promise of about
  95 from 0.3 to 1 um.
• Lenses Sol-gel AR, gt99 transmission from 0.3 to
  1 um.
• Ians design 6 mirrors, lt12 air-glass surfaces,
  1 dichroic mirror, 2 central obscurations.
• Optical throughput (excluding grating) 0.956
  x 0.9912 x 0.8 x .85 or 0.44

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Platform

• WFOS on TMT (CELT) (big and bigger!)

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Final Remarks

• Up coming activities
• Model the WFOS with the telescope designs
  proposed by System Eng. Group, mainly the Greg
  F/1.
• Determine set of specifications for the WFOS
  3-mirror model (possible variation on
  coll/camera, small matrix)
• Image quality - Dimensions, mass
• Resolution - Cost
• Max field - Tolerances
• Throughput - Other?
• Availability of components Large optics,
  coatings, gratings ( tiling), VPH, ADC, CCD.

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Final Remarks

• Conclusions (Ian Powell)
• It appears possible using the mirror-based
  approach to design optics for a four channel
  spectrograph instrument for an F/15
  Ritchey-Chretien telescope with an F/1.5 primary
  with adequate performance.
• It would appear that this approach could be
  extended to handle F/12 and F/20 Ritchey-Chretien
  arrangements.
• Extending it to handle an F/15 Gregorian
  telescope has revealed a residual tilt of around
  5 degrees of the image at the focal plane of the
  3 mirror anastigmat. Image quality is
  compromised as the F/ of the primary is reduced
  from F/1.25 to F/1 and the F/ of the telescope
  is increased from F/15 to F/18